1. Field of the Invention
The present invention relates to a dynamic pressure-type hydrodynamic bearing device, as well as a spindle motor and information device using the same.
2. Description of the Prior Art
A hydrodynamic bearing device comprises a shaft and a sleeve that supports the shaft, and a dynamic pressure-generating liquid that is interposed in the gap between the two parts. With rotation of the shaft, the dynamic pressure-generating liquid is gathered up by dynamic pressure-generating grooves that are formed on the shaft or sleeve, and generates pressure such that the shaft is supported within the sleeve without coming into contact therewith. As a result, high-speed rotation can be attained, and ambient noise during the rotation can be alleviated.
A spindle motor equipped with such a hydrodynamic bearing device can provide the requisite rotational accuracy with an increased recording density of the medium, and can furthermore provide excellent shock resistance and quietness. Thus, it can be used in a majority of motors for application in such representative magnetic disk devices as information technology equipment and audio-visual equipment.
In recent years, magnetic disk devices have become progressively miniaturized and have enhanced speed, and there is also a demand for the spindle motor, which is the main component, to be miniaturized and enhanced in speed.
In a conventional hydrodynamic bearing, air and liquid such as lubricating oil and the like have been used as the fluid for generating dynamic pressure in the gap between the shaft and the sleeve. However, the viscosity of air is low and requires extremely thin gaps and broad facing surfaces in order to generate pressure in the gap, and to achieve this requires highly precise machining. The cost of manufacturing increases as a result. Moreover, if liquid such as lubricating oil (referred to below as “dynamic pressure-generating liquid”) is used, in order to achieve the ideal low viscosity liquid that will reduce the torque (turning force) of a hydrodynamic bearing device, in other words to reduce the consumption electric power, it is necessary to reduce the vaporization loss that accompanies heat generation during continuous use.
Conventionally, hydrodynamic bearing devices have been proposed that use esters such as dioctyl sebacate (DOS), dioctyl azelate (DOZ), and dioctyl adipate (DOA) as dynamic pressure-generating liquid. Hydrodynamic bearing devices that use a mixture of neopentyl glycol esters of caprylic acid and capric acid (see for example Japanese published unexamined application No. 2000-336383), hydrodynamic bearing devices that use esters obtained from neopentyl glycol and C-6 to C-12 monovalent fatty acids and/or their derivatives as dynamic pressure-generating liquid (see for example Japanese published unexamined application No. 2001-316687), and hydrodynamic bearing devices that use polyol esters derived from diol components that have alkyl side chains at the β- or β,β′-positions (see for example Japanese published unexamined application No. 2002-195252) have been proposed.
However, while these conventional dynamic pressure-generating liquids of the hydrocarbon series have low viscosity and can reduce torque, their heat resistance is lower (higher vapor pressure) due to the characteristics of their molecular structure, and the amount of evaporation produced will increase as they reach a high temperature. For this reason, if used continuously in a high temperature environment, the amount of dynamic pressure-generating liquid required for the rotation of the bearing cannot be maintained.
For the amount of evaporation of the dynamic pressure-generating liquid considered previously, the above requirement makes it necessary to fill with an excess of the dynamic pressure-generating liquid, but the space and location required in order to fill with an excess of the dynamic pressure-generating liquid result in the miniaturization of the hydrodynamic bearing device and reductions in the cost becoming difficult.